4th gear problems

[cass.t25]

Hi, we've recently returned from a trip to cornwall in our 1.9 diesel 1983 t25. She runs lovely apart from on motorways/dual carriage ways at any speed over 55/60 it jumps out of fourth gear which is a bit scary when trying to overtake lorrys etc. We took it to a vw mechanic in cornwall who after a 5 min "inspection" said it needs a new gearbox. Anyone got any thoughts? . Also anyone know where i could sorce a recon gearbox if needed we live in Oxford. Thanks all